Method and apparatus for operating a power output stage

ABSTRACT

The invention relates to a method for operating a power output stage ( 2 ), in particular an electrical machine, which comprises a bridge circuit ( 3 ) with at least one half-bridge ( 1 ), wherein the half-bridge ( 1 ) has two power semiconductor switches ( 4, 5 ) which are connected in series and which are each supplied with an operating control voltage during normal operation in order to set a power voltage, and wherein the power voltage (u4, u5) of the respective power semiconductor switch ( 4, 5 ) is detected for the purpose of short-circuit monitoring. In this case, provision is made, in order to check the functioning of the short-circuit monitoring means in a check mode for the power semiconductor switches ( 4, 5 ), for a test control voltage which is below the operating control voltage to be set at least temporarily at the same time.

BACKGROUND OF THE INVENTION

The invention relates to a method for the operation of a power outputstage, in particular of an electrical machine, which comprises a bridgecircuit having at least one half-bridge, wherein the half-bridge has twoseries-connected power semiconductor switches which, in a normaloperating mode, are supplied with an operating control voltage in orderto set a power voltage, and wherein the power voltage of the respectivepower semiconductor switch is detected for the purpose of short-circuitmonitoring.

The invention also relates to an apparatus for the operation of a poweroutput stage, in particular of an electrical machine, in particular forperforming the abovementioned method, having a bridge circuit whichcomprises at least one half-bridge, wherein the half-bridge has twoseries-connected power semiconductor switches which, in a normaloperating mode, are supplied with an operating control voltage in orderto set a power voltage, and having a monitoring device which detects thepower voltage of the respective power semiconductor switch for thepurpose of short-circuit monitoring.

Methods and apparatuses of the type mentioned at the outset are knownfrom the prior art. In drive apparatuses for motor vehicles comprisingan electric drive, for example hybrid drive apparatuses or driveapparatuses for vehicles driven only electrically, power output stagesare usually provided for controlling the respective electrical machine,said power output stages comprising power semiconductor switches such asMOSFETs (metal-oxide semiconductor field-effect transistors) or IGBTs(insulated-gate bipolar transistors) and correspondingly associatedfreewheeling diodes. In order to control polyphase, permanently and/orseparately excited electrical machines, one half-bridge of the poweroutput stage is usually assigned to each phase. The respectivehalf-bridge comprises two series-connected power semiconductor switcheswhich, in a normal operating mode, are supplied with an operatingcontrol voltage in order to set a desired power voltage.

In order to ensure operating safety, it is also known to monitor thepower voltage of the respective power semiconductor switch or at oracross the respective power semiconductor switch in order to be able todetect a short-circuit situation. If one of the power semiconductorswitches goes into short circuit owing to a failure, then the logic ofthe half-bridge can no longer maintain the nominal pulse patternpredefined for the electrical machine for the output of torque; inparticular, if the second series-connected power semiconductor switchswitches despite the first power semiconductor switch beingshort-circuited. In this situation, a current flows which is generallyimpermissibly high and, in the worst case, leads to fires in the powercircuit since the electrical energy which is usually switched by thepower semiconductor switches is very high and the voltage range is solarge that arcs possibly remain for a long time. Preferably, the drivercircuit of the two power semiconductor switches monitors the respectivepower voltage. If said power voltage is too high in the switched-onstate, it is assumed that an overcurrent is flowing and the entiresemiconductor bridge is switched into freewheeling mode by switching-offor deactivating the power semiconductor switches. This is also known asdesaturation monitoring or Dsat monitoring. In this case, use is made ofthe property of the power semiconductor switches whereby they cause adisproportionate increase in the power voltage when the current is toolarge, the so-called desaturation current. The control voltage providedfor the switches is normally high enough that voltages which are assmall as possible are dropped in the event of operating currents in thenominal range of the half-bridge or the inverter, that is to sayconsiderably below the desaturation current, as a result of whichswitching and on-state losses of the semiconductors are reduced orminimized.

For the safety of the drive apparatus, it is now essential that saidapparatus should only go into operation when it is ensured that nouncontrolled torque changes which could make the vehicle go out ofcontrol act on the drivetrain. Therefore, safety-related hidden faultsmust be ruled out or reduced to a tolerable residual risk throughtesting or redundancies. In the case of the desaturation monitoring,this is problematic since the proper function can only be determinedwhen the short-circuit event is artificially induced. Since theshort-circuit current or desaturation current is generally very muchhigher than the nominal current during normal operation, without otherprecautions, an artificially induced short-circuit would shorten theservice life of the electrical components involved and possibly lead toan uncontrolled change in torque.

SUMMARY OF THE INVENTION

The method according to the invention therefore provides that, in orderto check the function of the short-circuit monitoring, in a check modefor the two power semiconductor switches of the half-bridge, a testcontrol voltage, which is below the operating control voltage, is atleast intermittently set simultaneously. The power semiconductorswitches are therefore preferably supplied with a voltage supply whichis reduced to a value at which the voltage monitoring of the switchdrivers does not yet respond. Since the two power semiconductor switchesof the single half-bridge are supplied with the test control voltage atleast intermittently simultaneously, a short circuit of the half-bridgeis simulated. Furthermore, since the test control voltage is below theoperating control voltage, the short-circuit current produced in thiscase turns out to be lower than if the operating control voltage werepresent. The power semiconductor switches begin to desaturate within arelatively short time, as a result of which the short-circuit monitoringresponds quickly and eliminates the short circuit. The current flowingduring the short circuit is in this case preferably set by the selectionof the test control voltage to be low enough that the torque pulseproduced at the electrical machine as a result is so small that it issafely damped or removed by integration, in particular by the massmoment of inertia of a rotor of the electrical machine, and so noimpermissible torque reaches the drivetrain of the vehicle. The check ofthe function of the short-circuit monitoring is preferably performedwhen the electrical machine has not been brought into operation orshould not produce any torque.

Preferably, firstly one of the power semiconductor switches is suppliedwith the test control voltage and subsequently the other powersemiconductor switch. In this case, the supply of the one powersemiconductor switch with the test control voltage is expedientlymaintained until the other power semiconductor switch has been at leastintermittently supplied simultaneously with the test control voltage,since the short circuit is simulated by the simultaneous presence of thetest control voltage. Expediently, the voltage monitoring is performedat both power semiconductor switches at the same time or firstly onlyfor one and subsequently for the other of the two power semiconductorswitches.

Particularly preferably, metal-oxide semiconductor field-effecttransistors (MOSFETs) or insulated-gate bipolar transistors (IGBTs) areused as power semiconductor switches. Preferably, the operating controlvoltage is given as an operating gate voltage and the test controlvoltage is given as a test gate voltage in order to set a drain-sourcevoltage as the power voltage. The control voltage is thereforeexpediently a gate voltage of the respective power semiconductor switch.

Preferably, the test gate voltage is selected such that the powersemiconductor switches desaturate at currents slightly above a nominalcurrent. The nominal current is the current which would actually be setby the applied test gate voltage if no short circuit were present. Asthe test gate voltage was selected as described above, it is ensuredthat the short-circuit monitoring starts quickly and the flowingcurrents are not dangerous for the electrical components.

Particularly preferably, the switch-on time of the power semiconductorswitches is selected such that the flowing current remains below 2.5times the nominal current. Assuming standard components, the powersemiconductor switches begin to desaturate in the range after 1 μs atthe latest. This means that the short-circuit monitoring is preferablyset such that it responds shortly afterward and eliminates the shortcircuit. The check or test ends after approximately 5 μs, in particularif prior art components are used.

It is also provided that, during the check, the electrical machine atstandstill is switched into a safe state, preferably by means of aninverter. The inverter is expediently an inverter having thehalf-bridge. In this case, the control of the inverter can be adapteddepending on the type of machine.

During the check, an energy source is preferably connected to thehalf-bridge via a pre-charging resistor to increase safety. By way ofexample, the energy source is a traction battery or another energystore. The connection via the pre-charging resistor prevents largecurrents from being able to flow for a relatively long time in theinverter or the half-bridge. The pre-charging resistor is also used toensure that, when an operating voltage is applied, the charging currentof an intermediate-circuit capacitor of the power output stage does notreach values which are too high and that oscillations of the drainvoltage do not occur owing to the low-inductance construction requiredby the system.

The apparatus according to the invention is distinguished by the factthat, in order to check the function of the monitoring device, in acheck mode for the power semiconductor switches, a test control voltage,which is below the operating control voltage, is at least intermittentlyset simultaneously. Preferably, the check is performed by a drivercircuit assigned to the respective power semiconductor switch.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail below on the basis ofan exemplary embodiment. In this respect, the single figure shows asimplified illustration of a half-bridge.

DETAILED DESCRIPTION

The figure shows a half-bridge 1 of a power output stage 2 (not shownmore specifically here) of a drive apparatus which comprises at leastone electrical machine as drive unit, which electrical machine iscontrolled by means of the power output stage 2, more precisely by meansof a bridge circuit 3 having the half-bridge 1. Expediently, the bridgecircuit 3 comprises a plurality of such half-bridges 1, which are ineach case associated with one phase of the electrical machine. Thehalf-bridge 1 has two series-connected power semiconductor switches 4and 5, which are preferably designed in each case as a MOSFET or IGBTswitching element. Each of the power semiconductor switches 4, 5 has afreewheeling diode 6 or 7 associated with it in the usual manner. Thehalf-bridge 1 is assigned to a certain phase or a winding phase of theabovementioned electrical machine, wherein the winding phase is presentat the Phx terminal, which is connected between the source terminal ofthe power semiconductor switch 4 and the drain terminal of the powersemiconductor switch 5 and between the freewheeling diodes 6 and 7. Thehalf-bridge 1 is supplied by means of a corresponding operating voltageU_(B), which is present at the drain terminal of the power semiconductorswitch 4, while the source terminal of the power semiconductor switch 5is connected to ground GND. For reasons of clarity, the other switchingelements present in an inverter are not illustrated. The half-bridge 1,as a part of the bridge circuit 3, forms a component of an inverter 8(not illustrated in more detail here) which generates the pulse patternsfor the electrical machine.

The power semiconductor switches 4 and 5 are operated or controlled viain each case one predefinable gate voltage G₄ and G₅, respectively, ascontrol voltage. As a result, the logic of the pulse pattern of thehalf-bridge 1 or the inverter 8 is likewise predefined. The half-bridge1 enters a critical state if one of the elements of the circuit, inparticular one of the power semiconductor switches 4, 5, goes into shortcircuit as a result of a failure. Assuming that the power semiconductorswitch 4 fails, the potential U_(B) is then constantly present at thewinding phase of the electrical machine or at Phx. The logic of theinverter 8 can now no longer maintain the nominal pulse pattern whichcauses the electrical machine to output a torque; in particular, if thedriver circuit switches on the power semiconductor switch 5 (low side)despite the short-circuited power semiconductor switch 4 (high side). Inthis case, a generally impermissibly high current flows from U_(B) toGND. In the worst case, this can lead to fires in the power circuitsince the electrical energy which is available at U_(B) is very high andthe voltage range is so large that arcs can possibly remain for a longtime, that is to say that the arc voltage is smaller than U_(B). Inorder to avoid this, the driver circuit (not illustrated in more detailhere) of the two power semiconductor switches 4, 5 monitors thedrain-source voltages U₄ and U₅, respectively, as the respective powervoltage. If these are too large in the switched-on state, an overcurrentor desaturation current is assumed and the entire half-bridge 1 isswitched into freewheeling mode. This so-called desaturation monitoringmakes use of the property of the power semiconductor switches 4, 5whereby they have a disproportionate increase in the voltages U₄, U₅when the saturation current is too large. In order to operate thehalf-bridge 1, preferably a gate voltage is in each case made availableto the power semiconductor switches 4, 5 in the normal operating mode,said gate voltage being high enough that as far as possible very smallvoltages in the event of operating currents in the nominal range of theinverter 8, that is to say considerably below the desaturation range, inorder to minimize switching and on-state losses of the semiconductorelements.

In order to ensure safety, a monitoring device, which monitors the powersemiconductor switches 4, 5 for short circuits, is provided. In order tobe able to ensure safety during operation, the short-circuit monitoringis preferably checked for operability when the electrical machine is atstandstill, and so it can be ensured that, if a short circuit doesactually occur as a result of a failure, this can be detected andremedied by means of the short-circuit monitoring device, by the powersemiconductor switches being switched off.

The monitoring device (not illustrated in more detail here), which ispreferably integrated in the driver circuit, supplies the powersemiconductor switches 4, 5 at least intermittently simultaneously witha test gate voltage, which is below the operating gate voltage, in orderto check the short-circuit monitoring. According to the presentexemplary embodiment, the gate test voltages are reduced to a value atwhich the voltage monitoring of the driver circuit does not yet respond.The values in this case are chosen to be low enough that the powersemiconductor switches 4, 5 already desaturate at currents slightlyabove, that is to say—depending on the application—for example startingfrom double, the nominal current. Preferably, the power semiconductorswitches 4, 5 are successively switched into short circuit by first oneand then the other power semiconductor switch being supplied with thetest gate voltage. In this case, the switch-on time of the powersemiconductor switches 4, 5 is selected such that the current wouldremain below 2.5 times the nominal current. Conventional semiconductorelements then begin to desaturate approximately one microsecond (1 μs)later. That means that the desaturation monitoring is set such that itresponds shortly afterward and eliminates the short circuit. The checkthen ends after five microseconds (5 μs) at most, as long as standardcomponents are used. During the check, the electrical machine of thedrive is preferably at standstill and is switched into a safe state bymeans of the inverter 8. The torque pulse occurring during the check issmall enough that it is safely damped or removed by integration by themass moment of inertia of the rotor of the electrical machine, and so noimpermissible torque reaches the drivetrain of the vehicle. Preferably,the energy drawn from the intermediate circuit during the check of allof the phases is limited, depending on the application, to 1 Ws or atleast to a value smaller than 1 Ws, in particular smaller than 3 Ws.

Preferably, at least during the check, a traction battery is connectedto the half-bridge 1 via a pre-charging resistor to improve safety andso said traction battery provides the operating voltage U_(B). Inaddition, another energy provider or energy source can be connected viathe pre-charging resistor. As a result of this, no large currents flowin the inverter 8 for a relatively long time. Furthermore, it ispreferred that the pre-charging is interrupted as soon as voltages ofbetween 20 volts and 80 volts, in particular between 30 volts and 65volts, are reached in the intermediate circuit of the inverter. Saidvoltages are enough to perform the abovedescribed check of theshort-circuit monitoring. In this way, the loading of the individualcomponents is further reduced.

The above described method therefore enables a check of a short-circuitmonitoring without high short-circuit currents, which could lead todamage to electrical components, being produced. No dangerous, inparticular uncontrolled, torques are introduced into the drivetrain of amotor vehicle having the power output stage 2. The power output stage 2expediently has a plurality of such half-bridges in order to form thebridge circuit 3. For reasons of clarity, reference is made below toonly one of said half-bridges, wherein the half-bridges are identical toone another. By means of the above described method, it is possible todiscover any hidden fault before the start-up of the electrical machine.In this case, no cost-intensive redundancies of the switch elements arenecessary. Furthermore, the above described method can also beretrofitted at relatively low expense in existing inverters.

The invention claimed is:
 1. A method for the operation of a poweroutput stage of an electrical machine, the power output stage includinga bridge circuit having at least one half-bridge, wherein thehalf-bridge has two series connected power semiconductor switches, themethod comprising: in a normal operating mode, supplying each powersemiconductor switch with an operating control voltage in order to set apower voltage; in a short-circuit monitoring mode, detecting the powervoltages of each respective power semiconductor switch, and determiningwhether a short-circuit condition exists based on the power voltages;and in a check mode for the power semiconductor switches, settingsimultaneously a test control voltage at each of the power semiconductorswitches, which is below the operating control voltage; wherein theoperating control voltage is an operating gate voltage, the test controlvoltage is a test gate voltage and the power voltage is a drain-sourcevoltage.
 2. The method as claimed in claim 1, wherein one of the powersemiconductor switches is supplied with the test control voltage andsubsequently the other power semiconductor switch is supplied with thetest control voltage.
 3. The method as claimed in claim 1, wherein atleast one from the group comprising metal-oxide semiconductorfield-effect transistors and insulated-gate bipolar transistors are usedas power semiconductor switches.
 4. The method as claimed in claim 1,wherein the test gate voltage is selected such that the powersemiconductor switches desaturate at currents slightly above a nominalcurrent.
 5. The method as claimed in claim 4, wherein the test gatevoltage of the power semiconductor switches is selected such thatcurrent during the check mode remains below 2.5 times the nominalcurrent.
 6. The method as claimed in claim 2, wherein the short-circuitmonitoring mode is performed while the electrical machine is atstandstill, and wherein, the electrical machine is switched into a safestate by means of an inverter.
 7. An apparatus for the operation of apower output stage, the apparatus comprising: a bridge circuit includingat least one half-bridge, the half-bridge having two series-connectedpower semiconductor switches which, in a normal operating mode, aresupplied with an operating control voltage in order to set a powervoltage; a monitoring device which detects the power voltage of therespective power semiconductor switch in a short-circuit monitoringmode; and a controller configured to check the function of themonitoring device, in a check mode by using an intermittent test controlvoltage, which is below the operating control voltage, simultaneouslyapplied to the power semiconductor switches; wherein the operatingcontrol voltage is an operating gate voltage, the test control voltageis a test gate voltage and the power voltage is a drain-source voltage.8. The method as claimed in claim 1, wherein the power output stage is apower output stage of an electrical machine.
 9. The apparatus as claimedin claim 7, wherein the power output stage is a power output stage of anelectrical machine.